The invention relates to a plasma detector and a laser beam machine with a plasma detector. Particularly, the invention relates to a plasma detector and a laser beam machine with a plasma detector for easily detecting plasma harmful to machining, having no complex structure.
Such kind of known conventional plasma detector measures electric resistance between a sensor electrode provided at a top end of a torch and a workpiece, and detects plasma generating from the workpiece on the basis of the measured electric resistance. When generating plasma, the voltage of a sensor electrode (“the trace voltage” hereinafter) fluctuates irrespective of a gap length between the torch and the workpiece, so that trace control does not nomally function. And, the portion between the sensor electrode and the workpiece is filled with charged particles comprising the generated plasma. For this reason, the above-mentioned electric resistance shows a predetermined value, being changed from open state (infinity). Then, plasma can be easily detected by watching the electric resistance. When thus detecting plasma, for instance, a feeding speed of the torch is decreased in order to restrict generation of plasma, so that the trace control can stably function.
In order to measure the electric resistance between the sensor electrode and the workpiece, it is necessary to provide a plasma detector with a circuit for measurement of electric resistance. If provided, the plasma detector has a complex structure, inconveniently.
It is also possible to detect plasma by watching the trace voltage without measuring the electric resistance between the sensor electrode and the workpiece. But, it is difficult to easily detect plasma harmful to machining only by watching the trace voltage since the trace voltage at the time when generating plasma is small in its volume, and fluctuates even if no influence is given to machining. If this method is adopted, the feeding speed of the torch is frequently decreased, so that machining on the workpiece is not smooth.
Then, a plasma detector and a laser beam machine with a plasma detector, for easily detecting plasma harmful to machining, having no complex structure are still desired.